DESCRIPTION: Previous studies show that consumption of soy milk containing significant quantities of the phytoestrogens, daidzein and genistein, by females for one month, effectively reduces levels of endogenous ovarian hormones and adrenal androgen, which are recognized risk factors for breast cancer. A soya diet containing genistein and daidzein will be given to women during specific segments of the menstrual cycle to evaluate the value of these natural compounds in reducing breast cancer risk through suppression of ovarian hormones. The first study will include 15-30 regularly cycling women who will be studied over 17 months as they ingest one dose of soya isoflavones (>200 mg) on separate months at seven specific periods during the menstrual cycle. Pharmacologic test cycles will alternate with control cycles. Blood and urine samples will be analyzed for isoflavones and ovarian hormones. The Second study will include males who consume repeatedly a soya diet preferentially enriched with one isoflavone within a two month period. This study will investigate factors which increase the bioavailability of genistein and will determine if soya phytoestrogens affect gonadotropin levels in addition to steroid hormones. The purpose of the studies is to determine the most efficient timing of phytoestrogen administration and the minimum effective dose for future studies of the effects of soya phytoestrogens on endogenous markers for cancer risk in women.